Additive Manufacturing of CaO-Pt/Al₂O₃ Structured Catalysts for Cyclohexane Dehydrogenation

Abstract

The dehydrogenation of cyclohexane is of vital importance for the production of Nylon-6 and Nylon-66, as it enhances atom utilization efficiency. Ca-doped platinum catalysts have been employed in alkane dehydrogenation due to their ability to selectively activate C–H bonds while minimizing C–C bond cleavage. However, owing to their limited selectivity toward cyclohexene, Pt-Ca/Al₂O₃ catalysts have not been widely adopted in the field of partial dehydrogenation to alkenes. In this work, Al₂O₃ supports are fabricated using the direct ink writing (DIW) 3D printing technique, incorporating designed channels. After impregnation and calcination at 550 °C, the distribution of active species, surface acidity, and basicity are optimized, resulting in a cyclohexene yield of 8.9%. The cyclohexene yield and stability of the 3D-printed catalysts are significantly higher than those of the granular catalyst, attributed to enhanced heat and mass transfer performance facilitated by the internal channels.

1. Introduction

Cyclohexene is a key intermediate widely used in the production of Nylon-6 and Nylon-66, as well as other fine chemicals [1]. Traditionally, cyclohexene is synthesized industrially via partial hydrogenation of benzene; however, this process inherently produces a significant amount of cyclohexane as a byproduct. To utilize the byproduct, catalytic oxidation of cyclohexane has been developed as an alternative route for cyclohexene production. However, this approach not only leads to substantial CO_x emissions but also reduces atomic utilization efficiency [1]. In contrast, the non-oxidative dehydrogenation pathway converts cyclohexane into cyclohexene, benzene, and hydrogen—valuable intermediates directly applicable to nylon synthesis—thereby improving overall atom utilization efficiency [2].

Pt-based catalysts exhibit excellent thermal stability at temperatures exceeding 600 °C [3] and demonstrate the ability to selectively activate C–H bonds while minimizing C–C bond cleavage [4], highlighting their potential for catalytic dehydrogenation applications [2]. However, the strong interaction between the acidic sites of Pt-based catalysts and unsaturated hydrocarbon intermediates often leads to undesirable side reactions, such as hydrogenolysis, cracking, and coke formation [4,5], resulting in decreased olefin selectivity [3,6] and catalyst activity [7]. Meanwhile, the dehydrogenation of cyclohexane is a highly endothermic process, with the majority of heat absorption occurring during the initial conversion of cyclohexane to cyclohexene [8,9]. The elevated reaction temperatures required to overcome the thermodynamic barrier [3] may exacerbate issues such as coking and catalyst deactivation.

To enhance the activity of Pt-based catalysts, alkali and alkaline earth metals—such as K, Ca, Sn, and Mg—are employed to neutralize acidic sites, improve the dispersion of active species, and optimize the interaction between active components and supports [3,7,10,11]. Among these promoters, Ca demonstrates superior performance in enhancing activity, stability, and dispersion [7,10,12]. Furthermore, calcination temperature plays a critical role in strengthening metal-support interactions [13], modifying oxygen-containing functional groups and dispersion [14], isolating Pt nanoclusters [5], and increasing surface area [15], thereby contributing to enhanced catalytic activity and stability [2,16]. However, the studies mainly focus on the catalytic dehydrogenation for benzene and hydrogen production [7,17], the cyclohexene yield was not studied enough.

An ideal dehydrogenation catalyst should possess the ability to efficiently manage reaction heat, facilitate olefin desorption, rapidly deliver products, and stably perform dehydrogenation at high temperatures—posing new challenges for catalyst shaping, composition, doping, and calcination. Three-dimensional printing, also known as additive manufacturing, enables the fabrication of catalyst geometry and composition as designed, which has been employed to enhance catalytic performance. This technique offers a novel strategy to enhance mass and heat transfer within and around catalytic materials, thereby improving reaction efficiency and extending operational lifetime.

To the best of our knowledge, 3D printing has been applied to fabricate ZSM-5 [18,19,20,21], ZrO₂ [22], and Al₂O₃ [23] catalysts for oxidative dehydrogenation. The engineered porous structure and tailored wall thickness enhance catalytic activity, stability, and operational lifetime. Hanh et al. [24] and Grobmann et al. [25] reported methods for fabricating Pt/Al₂O₃ catalysts for the direct dehydrogenation of perhydro-dibenzyltoluene (18H-DBT) and perhydro-dibenzyltoluene, respectively. The significance of pore structure and calcination temperature was emphasized in these studies. However, the addition of promoters was not mentioned, and the application to cyclohexane dehydrogenation (CDH) has not been investigated.

In this work, a novel approach for fabricating Pt-Ca/Al₂O₃ catalysts with designed geometries and internal flow channels is proposed for cyclohexane dehydrogenation to address the bottlenecks in fabrication and application. Various calcination temperatures are employed to optimize nanoparticle dispersion and catalyst acidity. The fabricated structures are stacked in tubular reactors to catalyze cyclohexane dehydrogenation at different temperatures. The influences of pore structure, acidity, and dispersion on catalytic performance are investigated. Finally, a cyclohexene yield of 8.9% is achieved at 550 °C when the weight hourly space velocity (WHSV) is 2.08 h⁻¹.

2. Results and Discussion

2.1. Three-Dimensional Printing, Debinding and Pore Analysis

The mass loss behavior of the 3D-printed materials is shown in Figure 1a. Two distinct mass loss events are observed in the temperature ranges of 50–180 °C and 280–600 °C, respectively, with maximum mass loss occurring at 320 °C. The first event corresponds to the release of adsorbed water, while the second reflects the degradation of organic components and the interaction of bentonite with atmospheric oxygen [26,27]. Therefore, a slow heating rate (2 °C min⁻¹) is applied to prevent structural fracture or collapse during thermal treatment [28]. When a temperature above 600 °C is applied, no distinguishable mass loss is observed, indicating complete decomposition of the organic components [29]. Hence, the calcination temperature is set at 600 °C, where the residual weight remains approximately 52.6% of the original value.

Figure 1. 3D-printed catalyst after calcination. (a) Thermogravimetric (TG) curve of the 3D-printed precursor (red: mass loss; blue: differential thermogravimetry); (b) The 3D-printed structure after impregnation and calcination; (c–e) Scanning electron microscopy (SEM) images of the 3D-printed catalyst; (f) Transmission electron microscopy (TEM) images of the 3D-printed catalyst.

The morphology of the 3D-printed catalyst after calcination and deposition of the active species (Pt and Ca) is presented in Figure 1b, indicating that no significant deformation occurred during the pyrolysis and impregnation processes. After impregnation and pyrolysis, the color of the structural surface changed from white (Figure S1d) to gray (Figure 1b). The scanning electron microscopy (SEM) image (Figure 1c) shows that the wall thickness is about 1.4 mm, the distance between the struts is approximately 3 mm, and the length has been reduced by about 6%. The rough surface of the structure indicates that numerous small pores are present within the struts. The close-up view (Figure 1d,e) reveals that the pore size within the struts is approximately 1–100 μm. These pores are attributed to the organic binders, which are burned off during calcination, thereby creating space for gas release. The transmission electron microscopy (TEM) image (Figure 1f) shows that the pores at the 10-nanometer scale, resulting from the crystalline structure, are uniformly distributed in the catalyst.

The nanopores were further characterized using the N₂ adsorption–desorption technique, as illustrated in Figure 2a. The samples were designated according to their respective calcination temperatures, with pure Al₂O₃ sample referring to the powders obtained from the direct calcination of pseudo-boehmite at 600 °C in the absence of bentonite. All six samples exhibit mixed type II and type IV isotherms with H₄-type hysteresis, according to IUPAC classifications [18,19]. The rise of adsorption isotherms at low relative pressure (P/P₀ < 0.03) corresponds to microporous adsorption, while the hysteresis loops in the P/P₀ range of 0.6–0.9 indicate capillary condensation occurring within mesopores, macropores, and on the external surface [30,31]. Compared with pure Al₂O₃, distinct shrinkage in the hysteresis loop is observed for the 3D-printed catalyst, which can be attributed to the incorporation of bentonite and the pore blockage caused by the loaded active species, such as calcium oxide [32]. The five samples calcined at different temperatures exhibit similar hysteresis loops, owing to identical loading contents. The slight reduction in adsorption and desorption capacities observed from the samples calcined at 650 °C and 750 °C can be attributed to the collapse and structural transformation of the support, which is promoted by the presence of CaO. The porosity characteristics are further supported by the samples’ pore size distributions (Figure 2b). For all the 3D-printed samples, micropores are predominantly concentrated at 1.1 nm and mesopores at 7.5 nm, with pore sizes smaller than those observed in the sample without the addition of bentonite or active species. The samples calcined at 650 °C and 750 °C exhibit larger pore diameters, indicating the occurrence of structural collapse or transformation [19,29], which is consistent with the results of isotherm analysis.

Figure 2. Nanopore analysis of the catalyst calcined at different temperatures. (a) Nitrogen physisorption isotherms; (b) Pore size distributions; (c) Brunauer-Emmett-Telle (BET) surface areas; (d) Pore volumes.

The derived surface area (S_BET) and total pore volume (V_p) are plotted in Figure 2c,d. For the directly calcined pure pseudo-boehmite at 600 °C, S_BET and V_p are 220.71 m² g⁻¹ and 0.74 cm³ g⁻¹, respectively. Following 3D printing, active species loading, and sintering, both S_BET and V_p decreased. The values further declined with increasing calcination temperature, which is consistent with the previous analysis.

2.2. Analysis of Crystalline Structure

This section presents an analysis of the elemental composition and crystalline structure evolution of the catalysts calcined at various temperatures, based on EDS, XRD, and TEM characterizations.

Figure 3a shows the elemental composition of the 550 sample as determined by inductively coupled plasma optical emission spectrometry (ICP-OES). The total weight fraction of metallic elements is approximately 40.11 wt%, with the remainder attributed to oxygen. The Pt and Ca weight fractions are approximately 0.47 wt% and 0.49 wt%, respectively, and the atomic ratio of Ca to Pt is about 5.08, indicating successful loading of the active species. The main elements, Si and Al, originate from pseudo-boehmite and bentonite. Fe, Mg, and Na are inherent components of bentonite, while other elements are trace impurities with mass fractions below 0.005 wt%. Since all catalysts use the same support, the influence of these additional elements on the catalytic performance is negligible. The atomic ratios of Ca and Pt were further analyzed by energy-dispersive X-ray spectroscopy (EDS) spot scanning and are presented in Figure 3b. The measured contents of Ca and Pt are approximately 87.7% and 18.27%, respectively, which are consistent with the ICP-OES results. To further investigate the distribution of the impregnated elements, elemental mapping was performed, as shown in Figure 3c–f. As illustrated in Figure 3d, calcium and platinum are uniformly distributed within the porous structure, and the particle size of the active species is significantly smaller than one micrometer, as confirmed by the images in Figure 3e,f.

Figure 3. Compositions of the 3D-printed catalyst. (a) Elemental composition of the catalyst determined by ICP-OES; (b) Elemental ration from EDS spot scanning; (c–f) Mapping scanning images.

The crystalline structure was analyzed by X-ray diffraction analysis (XRD) and is presented in Figure 4a. The peaks observed at approximately 39°, 46°, and 68° exhibit overlapping diffraction features. Upon closer examination, the earlier set of diffraction peaks at 39.49°, 45.86°, and 67.03° can be indexed to the (222), (400), and (440) crystal planes of γ-Al₂O₃ micro-crystallites (PDF: 97-006-8771), respectively [17,29,33,34]. The subsequent peaks at 39.98°, 46.67°, 68.15°, as well as those at 81.3° and 85.7°, are assigned to the (111), (200), (220), (311), and (222) crystal planes of metallic platinum (PDF: 03-065-2868), respectively [35,36,37,38]. As the temperature increases, the intensity of the diffraction peaks gradually rises, indicating that the proportion of γ-Al₂O₃ in the crystalline phase increases [17,29,33,34]. When the calcination temperature is below 550 °C, the diffraction peaks of Pt are inconspicuous, suggesting that the Pt particles are uniformly dispersed within the porous support at temperatures below 550 °C [39]. However, when the calcination temperature exceeds 550 °C, distinct Pt peaks are observed, demonstrating that partial agglomeration occurs at and above 550 °C [40], as further confirmed by TEM analysis. The peaks at 27.9°, 34.5°, and 54.4° correspond to the diffraction pattern of platinum dioxide (PDF: 97-000-4415), while the peaks at 36.1°, 59.8°, and 62.1° refer to the diffraction pattern of platinosic oxide (PDF: 97-003-0444). As the calcination temperature increases, the intensity of the PtO_x species decreases, indicating the thermal reduction of these oxides [38,41,42,43,44].

Figure 4. Nano-crystalline Structures of the catalyst sintered at temperatures ranging from 350 to 750 °C: (a) XRD pattern; (b–f) TEM images.

Moreover, the characteristic peaks corresponding to calcium-related crystalline phases in Figure 4a are weak, which can be attributed to the low content and uniform dispersion of calcium species on the support [45]. It should also be noted that the weak diffraction peaks observed at 22.3° and 32.9° suggest the presence of trace amounts of calcium aluminates (primarily Ca₄Al₆O₁₃, PDF: 97-001-6177) [11], the intensity of which increases with rising temperature. Additionally, the characteristic peaks at about 26° and 50° indicate the presence of a small amount of silicon dioxide (PDF: 97-003-9830) in the support, which originates from the bentonite.

The TEM images of the catalyst calcined at temperatures ranging from 350 °C to 750 °C are presented in Figure 4b–f. Figure 4b shows that the dark points are uniformly distributed on the support, indicating a particle size much smaller than 1 nm. The particle sizes for the 450 °C, 550 °C, and 650 °C samples are approximately 4 nm, 5 nm, and 7 nm, respectively (Figure 4c–e). For the 550 °C and 650 °C samples, the particle boundaries are slightly connected. When a higher calcination temperature is applied, the Pt particle boundaries clearly coalesce, forming particles larger than 10 nm (Figure 4f), suggesting aggregation of platinum nanoparticles [46]. This is because the consumption of CaO by Al₂O₃ at high temperatures leads to a decrease in the amount and size of CaO particles (Figure 4a) [11,47], which weakens the confinement effect of CaO toward Pt particles [48].

2.3. Analysis of Active Sites

To clarify the acidic and basic properties as well as the interaction between platinum and the support, the catalysts calcined at various temperatures were subjected to programmed-programmed desorption under NH₃, CO₂, and H₂ atmospheres.

As shown in Figure 5a, three distinct desorption peaks are observed in the temperature-programmed desorption of ammonia (NH₃-TPD) profile: the first peak below 200 °C corresponds to weak acidic sites, the second peak between 250 °C and 600 °C to medium-strength acidic sites, and the third peak above 600 °C to strong acidic sites [49,50]. The sample calcined at 350 °C exhibits the highest concentration of acidic sites, with four distinct ammonia desorption peaks observed at 175 °C, 300 °C, 425 °C, and 635 °C. As the calcination temperature increases (<550 °C), both the total acidity and acid strength of the supports decrease. This phenomenon can be attributed to the coverage of acidic sites by CaO following the gradual decomposition of Ca(NO₃)₂ as the temperature rises [51]. When the calcination temperature exceeds 550 °C, partial CaO is consumed by Al₂O₃ to form calcium aluminates (Figure 4a), resulting in enhanced sample acidity [11,52]. A greater number of strong acidic sites is observed in the 750 sample compared to the 650 sample. This phenomenon is attributed to the growth of platinum nanoparticles, which induces partial degradation of the support structure and consequently results in the re-exposure of certain acidic sites.

Figure 5. Programmed heating results under different atmospheric conditions. (a) NH₃-TPD profiles and the corresponding total acidity; (b) CO_2-TPD profiles and the corresponding total basicity; (c) H₂-TPR profiles and the corresponding H₂ consumption.

Figure 5b presents the temperature-programmed desorption of carbon dioxide (CO₂-TPD) profiles of the samples, with weak, medium-strength, and strong basic sites eluting below 250 °C, between 250 °C and 600 °C, and above 600 °C, respectively. The peak at 200 °C corresponds to the desorption from weak basic sites on the support, such as hydroxyl groups on Al₂O₃ [53], while the high-temperature desorption peak is associated with the formation of HCO₃⁻ or CO₃²⁻ species, contributing to strong basicity [54]. The sample calcined at 350 °C exhibits the lowest concentration of basic sites, displaying four distinct CO₂ desorption peaks observed at 200 °C, 450 °C, 575 °C, and 750 °C. As the calcination temperature increases, the desorption intensity gradually increases and reaches a maximum at 550 °C, corresponding to a total basicity of 256.79 μmol g⁻¹ (Figure 5b), indicating an increase in the number of basic sites. This trend can be attributed to the enhanced formation of CaO through the thermal decomposition of Ca(NO₃)₂ at elevated temperatures, which is consistent with the NH₃-TPD analysis [52]. Beyond 550 °C, however, the intensity decreases due to the formation of calcium aluminates, which consume available basic species.

The temperature-programmed reduction with hydrogen (H₂-TPR) profiles of the five catalysts are presented in Figure 5c. The profile of the catalyst calcined at 350 °C exhibits four reduction peaks at 110, 280, 610, and 850 °C, which correspond to the reduction of Pt–O–Pt species [44], the reduction of Pt supported on Al₂O₃ and CaO (second and third peaks) [2], and the reduction of the support surface [55], respectively. For samples calcined below 550 °C, the reduction temperature increases with rising calcination temperature due to strengthened interactions among Pt, the support, and CaO [56]. However, when the calcination temperature exceeds 550 °C, both the reduction temperature of Pt species on the support and that of the support itself decrease, suggesting the occurrence of hydrogen spillover effects [57,58]. The sample calcined at 550 °C exhibits the largest peak area compared to those calcined at 350 °C and 450 °C, indicating the highest concentration of active sites [2,3]. When a calcination temperature above 650 °C is applied, auto-reduction and aggregation of Pt particles occur, resulting in a reduced peak area [44]. The reduction temperature for metallic Pt in the catalyst should be below 400 °C [4]. In Figure 5c, the reduction temperature of Pt exceeds 400 °C, indicating the presence of Pt oxides. If all Pt were present in the oxidized state, the expected H₂ consumption would range from 26 to 51 μmol g⁻¹, corresponding to Pt²⁺ and Pt⁴⁺, respectively. However, the maximum H₂ consumption observed is 25.04 μmol g⁻¹ (for the 550 °C sample), which is lower than the theoretical minimum of 26 μmol g⁻¹, demonstrating that a portion of Pt is already in the metallic state. This result is consistent with the XRD analysis.

2.4. CDH Performances

Catalytic performances at different temperatures of the 3D printed Pt-Ca/Al₂O₃ catalysts are depicted in Figure 6a–c. As the reaction temperature increases, volcano-type curves with peaks at 450 °C are observed for all five catalysts (Figure 6a). The catalysts calcined at 450 °C exhibit higher conversion compared to the others, especially under elevated temperatures, due to the uniform distribution of platinum nanoparticles facilitated by the confinement effect of CaO, as revealed by TEM analysis [11]. The catalysts calcined at 350 °C exhibit the lowest conversion among the five samples, attributed to the highest concentration of acidic sites arising from incomplete decomposition, which promotes hydrogenolysis and carbon deposition [7,11]. It is noteworthy that the conversion of the 550 °C sample is lower than that of the 450 °C sample, despite its lower acidity and similar particle size. This is attributed to a greater amount of Pt being anchored on CaO, forming M2 sites (Figure 5c), which reduce conversion and enhance selectivity [3].

Figure 6. Catalytic performance of the Pt-Ca/Al₂O₃ catalysts calcined at various temperatures. (a) Cyclohexane conversion; (b) Selectivity of C₆H₁₀ (solid line) and C₆H₆ (dashed line); (c) Yield of C₆H₁₀ (solid line) and C₆H₆ (dashed line); (d) Stability test of the Pt-Ca/Al₂O₃ catalysts calcined at 550 °C (WHSV = 2.08 h⁻¹; N₂ = 150 mL min⁻¹).

As shown in Figure 6b, the selectivity toward cyclohexene increases with rising reaction temperature, as higher temperatures provide greater energy to promote the cyclohexene formation pathway [5]. The liquid by-products consist primarily of benzene, which can be used as a raw material for nylon production. The catalysts calcined at 350 °C exhibit significantly higher selectivity toward cyclohexene compared to the other catalysts. However, a very low conversion of the 350 °C sample is observed in Figure 6b. It can be deduced that the carbon formed as a result of acidity covers the active sites, leading to increased selectivity, which has no practical significance [3,59]. The catalysts calcined at 550 °C also exhibit higher selectivity toward cyclohexene, attributed to the enhanced desorption of cyclohexene facilitated by Ca, thereby suppressing further dehydrogenation [7,17]. Additionally, the high concentration of basic sites inhibits side reactions promoted by acidic sites [59]. Correspondingly, these catalysts achieve a cyclohexene yield of 8.9% (Figure 6c). The cyclohexene yield reported in previous studies [1,6] ranges from 3.8% to 6.0% at 550 °C under a gas hourly space velocity (GHSV) of 360 h⁻¹, which is lower than our yield of 8.9% achieved at the same reaction temperature but at a higher GHSV of 1887 h⁻¹. Additionally, it is noteworthy that the catalysts calcined at 450 °C show a stable benzene yield of approximately 90% over the reaction temperature range from 450 °C to 550 °C, which can be attributed to the coordination between acidic sites and nanoparticle dispersion.

The 3D-printing slurry was also manually shaped into granules with approximate dimensions of 3 mm × 1 mm (Figure S3) and prepared using the same protocol. A comparison of the dehydrogenation performance was conducted between the 3D-printed catalyst and the granular catalyst (Figure 6d). For the 3D-printed catalyst, the initial conversion decreases monotonically over the first 300 min, whereas the selectivity toward C₆H₁₀ increases. This phenomenon can be attributed to the in situ reduction and activation of the catalyst. After the reduction process, the catalyst exhibits stable conversion and selectivity toward cyclohexene. However, the granular catalysts exhibit significantly lower conversion, selectivity, and yield, all of which continuously decline until approaching zero (dashed line in Figure 6d). To maintain the same catalyst temperature (550 °C), the granular catalyst system requires a higher furnace temperature (approximately 590 °C), whereas the 3D-printed system achieves this at a lower furnace temperature (approximately 570 °C). Consequently, the lower conversion and selectivity can be attributed to the elevated temperature required to sustain the catalyst temperature [60]. Moreover, the higher furnace temperature accelerates catalyst deactivation, leading to a shorter operational lifetime, as illustrated in Figure 6d. The integrated channels within the catalyst also play a crucial role in facilitating product removal from the catalyst surface and enabling energy supply via mass flow [60,61], which warrants further investigation in future studies.

3. Materials and Methods

3.1. Additively Manufacturing of Catalyst

The powder mixtures (Figure S1a), consisting of 85 wt% pseudo-boehmite (>99.9%, Macklin, Heze, China), 10 wt% bentonite (RG, Macklin, Shanghai, China), and 5 wt% starch (AR, Macklin, Shanghai, China), were ground in a planetary ball mill (QM-WX4, Nanjing NanDa Instrument Plant) for 6 h at a rotational speed of 270 rpm. Subsequently, the resulting powders (100 g) were added to a solution (80 g) containing 2 wt% polyethylene glycol 400 (AR, Macklin, Shanghai, China) and 5 wt% HNO₃ (AR, Haohua, Luoyang, China) and stirred until a homogeneous slurry (Figure S1b) was formed.

The slurry was extruded via a DIW printer (Tronxy, Shenzhen, China) with a layer thickness of 150 μm to obtain the designed geometry (Figure S1c). The 3D-printed structures were then dried at 80 °C for 12 h and calcined at 600 °C in a muffle furnace (KEJING, Hefei, China) for 6 h to yield Al₂O₃ supports (Figure S1d). The resulting supports were subsequently impregnated sequentially with aqueous solutions of Ca(NO₃)₂ (AR, Adamas, Shanghai, China) and H₂PtCl₆ (AR, Adamas, Shanghai, China), followed by drying at 60 °C and calcination at 350, 450, 550, 650, or 750 °C for 4 h after each impregnation step to achieve a Pt content of 0.5 wt% at a Ca:Pt molar ratio of 5 [7,11].

3.2. Material Characterization

Thermogravimetry (TG) of the 3D-printed precursors was conducted in air atmosphere using a Q500 V20.10 Build 36 instrument, with heating performed from 30 to 1000 °C at a heating rate of 10 °C·min⁻¹. X-ray diffraction (XRD) analyses were carried out on a Shimadzu XRD-6000 diffractometer employing Cu Kα radiation (k = 1.5406 Å) at an operating voltage of 40 kV. The samples were ground into fine powders and scanned over a 2θ range of 10° to 90°. Inductively coupled plasma optical emission spectrometry (ICP-OES) was conducted using an Agilent 5110 OES (Agilent, Santa Clara, America) instrument to determine the elemental composition of the prepared catalyst, which was subjected to microwave-assisted digestion. The microstructural features of the porous architectures were examined by scanning electron microscopy (SEM, GeminiSEM 500, Oberkochen, Germany) and transmission electron microscopy (TEM, JEOL JEM-F200, Tokyo, Japan). Nitrogen physisorption measurements were performed using a Micromeritics ASAP 2460 (Norcross, America) analyzer to determine specific surface areas and pore size distributions. Prior to analysis, all samples were degassed at 300 °C for 8 h under vacuum. Temperature-programmed desorption of ammonia (NH₃-TPD), temperature-programmed desorption of carbon dioxide (CO₂-TPD), and temperature-programmed reduction with hydrogen (H₂-TPR) were performed using a Micromeritics ChemiSorb 2920 (Norcross, America) instrument. The heating rate was 10 °C min⁻¹ for all experiments, and the carrier gases were helium (He, 99.999%), helium (He,99.999%), and 10% H₂ in argon (Ar, 99.999%), respectively.

3.3. Evaluation of Dehydrogenation Performance

The CDH reactions were carried out in a vertical tubular reactor (Figure S2) with an internal diameter of 20 mm under atmospheric pressure. The 3D-printed Pt catalysts were loaded into the reaction zone, while the remaining sections were filled with 3D-printed supports. Five thermocouples were installed to monitor the temperatures at the inlet (1), reaction zone (2–4), and outlet (5), respectively. The catalyst was reduced in situ using cyclohexane at the operating temperature for 3 h. The pumped liquid cyclohexane (5.53 mL min⁻¹) was vaporized at 200 °C and carried into the reactor by N₂ (150 mL min⁻¹). The WHSV was determined to be 2.08 h⁻¹, while the gas hourly space velocity (GHSV) was calculated as 1887 h⁻¹, both based on the catalyst weight and the C₆H₁₂ feed flow rate. The product gas was analyzed using an online gas chromatograph equipped downstream with a flame ionization detector (FID) and a DB-35 column (Agilent). For all catalysts, cyclohexene (C₆H₁₀) and benzene (C₆H₆) were identified as reaction products. C₆H₁₂ conversion, C₆H₁₀ selectivity, and C₆H₁₀ yield were defined according to Equations (1)–(3), respectively.

$$C_6{H}_{12}conversion={\displaystyle \frac{C_6{H}_{12}(inlet)-C_6{H}_{12}(outlet)}{C_6{H}_{12}(inlet)}}\times 100\%$$

(1)

$$C_6{H}_{10}selectivity={\displaystyle \frac{C_6{H}_{10}(outlet)}{C_6{H}_{10}(outlet)+C_6{H}_6(outlet)}}\times 100\%$$

(2)

$$C_6{H}_{10}yield={\displaystyle \frac{C_6{H}_{10}(outlet)}{C_6{H}_{12}(inlet)}}\times 100\%$$

(3)

4. Conclusions

In conclusion, a novel approach for fabricating structured catalysts to enhance cyclohexene yield in cyclohexane dehydrogenation is presented. This method involves DIW 3D printing, impregnation, and calcination. Owing to the advantages of 3D printing, the resulting catalysts feature a hierarchical pore structure ranging from sub-2 nm micropores to designed channels of up to 1 mm, which significantly improves heat and mass transfer. However, the 3D printing process and the loading of active species may lead to a reduction in both pore size and pore volume. With increasing calcination temperature, the amount of CaO increases, the particle size of CaO decreases, and the extent of CaO consumption by Al₂O₃ rises, thereby affecting segregation behavior as well as surface acidity and basicity. Furthermore, the 3D-printed catalysts were calcined at various temperatures and subsequently packed into a tubular reactor to evaluate their performance in cyclohexane dehydrogenation. The catalyst calcined at 550 °C demonstrated superior conversion and selectivity toward cyclohexene, achieving a yield of 8.9%. Compared with conventional granular catalysts, the 3D-printed counterparts exhibit a more stable and higher cyclohexene yield, attributed to efficient product removal and an enhanced heat transfer surface area.